Conventionally, alkaline earth metal carbonate for a cathode ray tube has been synthesized by adding sodium carbonate aqueous solution or ammonium carbonate aqueous solution into a binary mixed aqueous solution comprising barium nitrate and strontium nitrate, or a ternary mixed aqueous solution comprising above-mentioned binary mixed aqueous solution and calcium nitrate, at a predetermined addition rate and reacting therewith to thus precipitate binary (Ba, Sr) carbonate or ternary (Ba, Sr, Ca) carbonate. The method includes, for example, a sodium carbonate precipitating method. This sodium carbonate precipitating method represents synthesizing alkaline earth metal carbonate by adding a sodium carbonate aqueous solution as a precipitant into a binary mixed nitrate aqueous solution comprising barium nitrate and strontium nitrate or a ternary mixed nitrate aqueous solution comprising barium nitrate, strontium nitrate and calcium nitrate. The method using the binary solution is shown in the following Chemical Formula 1 and the method using the ternary solution is shown in the following Chemical Formula 2. EQU (Ba, Sr)(NO.sub.3).sub.2 +Na.sub.2 CO.sub.3 .fwdarw.(Ba, Sr)CO.sub.3 +2NaNO.sub.3 Formula 1 EQU (Ba, Sr, Ca)(NO.sub.3).sub.2 +Na.sub.2 CO.sub.3 .fwdarw.(Ba, Sr, Ca)CO.sub.3 +2NaNO.sub.3 Formula 1
When the binary carbonate and ternary carbonate synthesized by the sodium carbonate precipitating method are analyzed by X-ray (wave length is 0.154 nm) diffraction analysis, the diffraction patterns are obtained as in FIG. 18 and FIG. 19. According to FIG. 18 and FIG. 19, there is observed to be one peak respectively in a part of the interplanar spacing ranging from 0.33 nm to 0.40 nm or in the part of a diffraction angle ranging from 22 to 27.degree. (the part between the two dotted lines in FIG. 18 and FIG. 19). The number of the peak does not change regardless of how the synthesizing condition such as reaction temperature or concentration of the aqueous solution or the like is changed during synthesis of carbonate. Moreover, if sodium carbonate is replaced by ammonium carbonate, the same result can be obtained.
Next, yttrium oxide is added into the above mentioned alkaline earth metal carbonate in an amount of 630 wt.ppm to make a mixture. Then, this mixture is dispersed into a solution in which a small amount of nitrocellulose is added into a mixture medium containing diethyl oxalate and diethyl acetate to make a dispersion solution. This dispersion solution is coated onto the cathode base and thermally decomposed in a vacuum to make an emitter for a cathode containing alkaline earth metal oxide as a main component. Then, the relation between the operating time and the emission current remaining ratio at the current densities of 2A/cm.sup.2 and 3A/cm.sup.2 are shown in FIG. 20. The line "a" represents the relation in the case where the binary carbonate is employed for an emitter and the current density is 2A/cm.sup.2. The line "b" represents the relation in the case where the ternary carbonate is employed for an emitter and the current density is 2A/cm.sup.2. The line "d" represents the relation in the case where the binary carbonate is employed for an emitter and the current density is 3A/cm.sup.2. The line "e" represents the relation in the case where the ternary carbonate is employed for an emitter and the current density is 3A/cm.sup.2. The emission current remaining ratio is the normalized value of the emission current with respect to the operating time based on the initial value of the emission current as 1 (the ratio of the emission current with respect to the operating time in the case of setting the initial value of the emission current as 1), and it can be said that the larger the emission current remaining ratio, the better the emission characteristic. As is apparent from FIG. 20, in the operations at the current density of 3A/cm.sup.2, the emission current remaining ratio is quite low in both binary and ternary carbonate. It can be said that the allowed value of the current density of these emitters is approximately 2A/cm.sup.2.
Recently, as a CRT has a larger screen size, higher brightness and higher resolution, the higher density of emission current has been demanded. However, if the conventional emitter materials for CRTs are used at the current density above 2A/cm.sup.2, a sufficient lifetime cannot be maintained. Thus, the conventional emitter materials cannot be employed for a CRT that is aiming at a larger screen size, higher brightness and higher resolution.